The goal of this proposal is to understand the multiple roles that the Notch signaling pathway plays during embryogenesis in mammals. The components of this pathway are highly conserved in evolution, suggesting that this pathway is fundamentally important for cell-cell interactions and the control of differentiation in multicellular organisms. Null mutations of three Notch family genes in mice (Notch1, Notch2, and int-3) have been constructed. Two of these genes (Notch1 and Notch2) are essential for normal embryonic development, confirming the importance of the Notch signaling pathway in mammals. In this proposal, a genetic approach will be used for the functional analysis of the Notch signaling pathway. The prediction that Notch family genes in mice share substantial functional redundancy will be tested by creating three different double mutant combinations, and assessing these double mutants for dosage-sensitive genetic interactions and synergistic effects of the mutations. This proposal will also test the hypothesis that the genes encoding ligands for the Notch family of receptors, like the genes encoding the receptors themselves, are essential for normal embryonic development. This hypothesis will be tested by creating a null mutation of the Jagged gene (a mouse homolog of the Notch ligand Serrate), analyzing the phenotype of homozygous Jagged null mutant embryos and mice, and analyzing interactions between the Jagged mutations and Notch mutations. These studies may further our understanding of the roles of the Notch signaling pathway in mammalian development.